3.1 Microstructure analysis
Figure 3 shows the morphology of as-deposited LDMed Ti-622222 alloy. It can be found in macrostructure (Fig. 3(a)) that the adjacent deposited layers are in metallurgical bonding state showing obvious layer band. The epitaxial growth coarse β columnar grains can also be observed, and the adjacent β columnar grains alternate between light and dark due to the different orientations of the grains in the structure . These β columnar grains basically grow along the deposition direction and slightly tilt in the scanning direction of laser beam. During the laser deposition process, the temperature gradient is usually along the deposition direction due to the lower temperature for the substrate. Therefore, the solidification process starts from the substrate side, and the nucleation grows mainly along the direction of the maximum temperature gradient according to the crystal growth theory . Moreover, the growth of β columnar crystals along the deposition direction is continuous as shown in Fig. 3(b), which is due to the deposition characteristic of layer by layer and the metallurgical bonding for interlayer for laser deposition manufacturing. It can be found in Fig. 3(c) that the fine basket-weave structure evenly distributes within the coarse β columnar crystals. The basket-weave structure consists of large numbers of disoriented and elongated α phases and a small amount of interphase β phases, which are staggered. The proportion of α phase is much higher than that of β phase. In addition, it can be observed that the grain boundaries are intact and there exists a large number of α lamella around the grain boundaries, which grow into clusters along the grain boundaries. During the laser deposition process, the formation of present deposited layer may produce the effect of reheat and even remelting on the last deposited layer. Thus, the internal structure for the LDMed Ti-622222 alloy is subjected to rapid heating and cooling for many times. In this case, the β phases in the microstructure rapidly transforms into α phases and the large numbers of primary α phases will be rapid growth with elongated shape. Therefore, the microstructure of the as-deposited sample is fine and the volume fraction of α phase is significantly greater than that of β phase [33, 34].
Figure 4 shows the microstructure of the LDMed Ti-622222 alloy after heat treatment. It can be seen that the microstructure of the heat-treated Ti-622222 alloy changes obviously compared with that of as-deposited alloy shown in Fig. 3(c). After the solution and aging treatment, the microstructure is still composed of the basket-weave structure knitted with α phase and β phase, but the phase morphology of LDMed Ti-622222 alloy change dramatically with the solution temperature increasing. This is mainly caused by the process of α + β/β phase transition at high temperature. It has been reported that  the content of comprising elements for Ti alloy can affects (α + β)/β phase transition point (Tα+β/β) according to the following formula:
T α+β/β=885 ℃+∑(Ci × ηi)
where Ci represents the content of i element, and ηi is on behalf of the effect of i element content on α + β/β phase transformation point. Based on the chemical component of Ti-622222 alloy listed in Table 1 and the measured ηi value , the value of Tα+β/β for Ti-622222 alloy calculated by the formula is about 940 ℃. When the solution temperature is 900 ℃ lower than the Tα+β/β, the solution process is carried out within the two-phase region of α + β. In this case, the partial primary α phases transforms into β phases. And then small amounts of elongated secondary α phases are precipitated in the β phase matrix in the subsequent aging process, as shown in Fig. 4(a). And due to the high temperature, the residual primary α phases grows obviously showing the shape of rod-like with lower length-width ratio. In the condition of HT-2, more primary α phase have transformed into β phases during the solution process, which finally leads to the decrease in the content of short rod-like primary α phases. Finally, more fine secondary α phases precipitate during the aging process, as shown in Fig. 4(b). When the solution temperature reaches the Tα+β/β (940 ℃), almost all the primary α phases transform into β phases, and then secondary α phases fully precipitate and grow up in the β phase matrix during the aging process. Therefore, the microstructure at room temperature is composed of elongated lamellar secondary α phases and interphase β phases as shown in Fig. 4(c).
3.2 Microhardness analysis
The average microhardness of LDMed Ti-622222 alloy with different solution and aging treatments are shown in Fig. 5. It can be seen that the solution and aging treatment has significant effects on the microhardness of LDMed Ti-622222 alloy. The average microhardness of the as-deposited Ti-622222 alloy is only 569.5 HV0.2, which is much lower than that of the heat-treated alloy. Moreover, the microhardness of LDMed Ti-622222 alloy shows a rising trend with the solution temperature increasing. It is mainly because that with the solution temperature increasing, more α phases transforms into β phases, which promotes the precipitation of secondary α phases with small size during the aging process (Fig. 4). The fine secondary α phases produce distinct fine-grained strengthening effect resulting in the microhardness increasing. In addition, in the condition of HT-3, the secondary α phases fully precipitate and grow into thin lamellar phases (shown in Fig. 4(c)) with poor compatible deformation capability compared with primary α phases, which may also result in the microhardness increasing of LDMed Ti-622222 alloy after HT-3 treatment.
3.3 Tensile property analysis
The tensile properties derived from the the stress-strain curves for LDMed Ti-622222 alloy before and after solution and aging treatment are shown in Fig. 6. It can be seen that for the as-deposited Ti-622222 alloy, the average ultimate strength (σb) is 1025 MPa and the average yield strength (σ0.2) is 943 MPa, which is higher than the tensile results for LDMed TC4 alloy reported by other literature . For plasticity, the elongation (δ) is 6.0% and the reduction of area (ψ) is 14.7%. By contrast, there is an obvious increase in strength for Ti-622222 alloy after the solution and aging treatment. Moreover, the strength increases with the solution temperature increasing. The LDMed Ti-622222 alloy after HT-3 treatment exhibits the highest σb value (1197.4 MPa) and σ0.2 value (1080.7 MPa). On the contrary, the plasticity (δ and ψ) decreases with the solution temperature increasing, and the optimum plasticity (δ: 7.8%; ψ: 19.2%) is obtained by the LDMed Ti-622222 alloy after HT-1 treatment.
The variation of tensile properties for LDMed Ti-622222 alloy closely depends on its microstructure. After solution and aging treatment, the primary α phase decreases but the fine secondary α phase increases (Fig. 4). The precipitation of fine secondary α phase leads to the increase in intergranular boundaries, which can hinder the dislocation slip improving the strength. Therefore, the strength for LDMed Ti-622222 alloy after heat treatment is higher than that of as-deposited alloy and increases with the solution temperature increasing. In addition, the plastic deformation ability of the primary α phase with short rod shape is stronger than that of the lamellar secondary α phase. Therefore, with the solution temperature increasing, the precipitation of lamellar secondary α phase increases leading to the decreasing of plasticity of LDMed Ti-622222 alloy. It is well known that there exists the anisotropy of tensile property in different sampling sections for LDMed samples due to the microstructure anisotropy . Therefore, it can be deduced that the LDMed Ti-622222 alloy in scanning direction exhibits higher strength but lower plasticity than that of sample in deposition direction due to the more grain boundary for β columnar grain in scanning direction (Fig. 3(b)) [28, 29].
Figure 7 shows the tensile fracture morphology of the LDMed Ti-622222 alloy before and after heat treatment. In macroscopic view (Fig. 7(a)-(d)), LDMed Ti-622222 alloys before and after heat treatment all show obvious cleavage appearance formed by the brittle fracture of α/β lamellae, which indicates the brittle characteristics corresponding to the low plasticity . By contrast, the tensile sample after HT-1 treatment exhibits obvious neck shrinkage (Fig. 7(b)) indicating the relatively superior plasticity. The micro-fractography is well related with the plasticity value for tensile property (Fig. 6(b)). In microscopic view (Fig. 7(e)-(h)), numerous fine dimples distribute on the fracture surface indicating that the LDMed Ti-622222 alloys are involved the ductile fracture mechanism . Generally, LDMed samples would inevitably produce certain hole defects after powders were melted and solidified, which may be caused by unmelted powder or gas. Previous report has shown that the existence of microvoids contributes to improve plasticity of the material . The analysis result of fracture morphology shows that the fracture mechanism of the LDMed Ti-622222 alloys is a mixture of ductile and brittle fracture.
3.4 Impact property analysis
Figure 8 shows the impact properties of LDMed Ti-622222 alloy. It can be seen that the average impact toughness of the impact sample in scanning direction is 23.4 J/cm2, lower than that in deposition direction (40.9 J/cm2). The obvious anisotropy of impact property depends on the microstructure characteristics in different sampling direction. During the impact process, the impact load is parallel to the growth direction of β columnar grain for the impact sample in scanning direction, but perpendicular to the growth direction of β columnar grain for the impact sample in deposition direction. Compared with the impact sample in scanning direction, the impact sample in deposition direction needs to overcome more grain boundaries during the fracture process, and then can consume more energy during the impact process. Therefore, the impact toughness of the sample in deposition direction is higher than that in scanning direction.
Compared with as-deposited samples, the impact toughness of the LDMed Ti-622222 alloy is improved after solution and aging treatment. With the solution temperature increasing, impact toughness first increases and then decreases. By contrast, the fluctuation of impact toughness for sample in scanning direction is larger than that in deposition direction. Toughness is the comprehensive expression of material strength and plasticity, which is closely related to the microstructure characteristics. After solution and aging treatment, the microstructure has been significantly refined due to the precipitation of lamellar secondary α phases (Fig. 4), which promotes the impact toughness increase. In addition, the heat treatment can eliminate the residual stress generated in the process of laser deposition and reduce the internal defect of the sample to some extent, which can also improve the impact toughness.
Figure 9 shows the impact fracture morphology of the LDMed Ti-622222 alloy in scanning direction before and after heat treatment. In macroscopic view (Fig. 9(a)-(d)), LDMed Ti-622222 alloys before and after heat treatment all show obvious cleavage appearance indicating the brittle characteristics. In microscopic view (Fig. 9(e)-(h)), numerous fine dimples distribute on the fracture surface indicating the ductile characteristics. In contrast, the fracture undulate is larger and the dimples are deeper for the impact samples after HT-1 and HT-2 treatment, indicating that the samples absorb more energy during the impact fracture process and have higher impact toughness.
Figure 10 shows the impact fracture morphology of the LDMed Ti-622222 alloy in deposition direction before and after heat treatment. Its variation tendency is in agreement with that in scanning direction (Fig. 9). Compared with impact samples in scanning direction, at the same solution temperature, the cleavage content is lower and the dimples of the impact samples in deposition direction are larger and deeper, which leads to the better toughness.
By comprehensive analysis of the impact performance data and fracture morphology, it is known that the specimen has relatively excellent impact performance when the solution temperature is 920 ℃.